A typical infra-red transmission systems includes a transmitter and a receiver. The transmitter has an optical source typically a GAs light emitting diode (LED) whilst the receiver utilizes a photodetector typically a PIN photodiode for converting incident optical power into an electric current.
Infra-red signals received by the receiver are very weak at a distance from the source, the output current provided by the photodetector being in the nano ampere range. It is therefore essential to provide a preamplifier in the front end to amplify the weak incoming signals to digital voltage levels.
The sensitivity of the receiver is limited by the noise sources at the front end and therefore it is important that the preamplifier be low noise. Furthermore, when a receiver is used in artificial light or sunlight, a fluctuating infra-red component will be present in the received signal and this must be removed by suitable electrical filtering.
There are two known approaches to the design of preamplifiers for infra-red receivers. In one such approach a high input impedance (HZ) amplifier is used. This amplifier tends to integrate the incoming signals in view of its high input impedance and associated stray capacitance and to compensate for this the preamplifier is followed by an equalizer which has a high pass filter characteristic. A known example of such an arrangement is the Siemens TDA 4050.
A problem arises with the above HZ amplifier approach in that the load for the photodetector diode is usually a RLC tuned circuit in order to enable the preamplifier to have the required band pass filter characteristic at the desired signal frequency. This however requires the use of a coil, which cannot be integrated and therefore when the preamplifier is manufactured as an integrated circuit chip, as is desirable, the coil must be provided as an additional off-circuit component. This coil needs elaborate shielding if installed in a harsh electromagnetic environment. Also a RLC circuit is a relatively selective band pass filter and hence pulse trains, as used in conventional infra-red modulation schemes tend to be stretched due to ringing. This further complicates the reception of comprehensive signals.
An alternative approach is to use a transimpedance TZ amplifier. This has a low input impedance and therefore the photodiode stray capacitance causes no serious signal integration. A problem is caused however by low frequency noise due to background infra-red radiation. The amplifier does not adequately reject this low frequency noise and this tends to upset the bias point at the input to the amplifier.
This invention seeks to provide a front end suitable for use in an infra-red receiver in which the above mentioned problems of known front ends are mitigated.